In prior art instruments used for the detection of plutonium-americium, various scintillation crystals have been used. These crystals have been mounted on a thick quartz light pipe which is attached to a photomultiplier tube. Europium activated calcium fluoride crystals have been used in these devices but have produced very poor results.
The instrument in present use consists of a thallium activated, sodium-iodide scintillation crystal mounted on a thick quartz light pipe which is attached to a photomultiplier tube. The components are sealed in a stainless-steel case with a beryllium window.
These instruments have failed due to cracked and hydrated crystals, defective photomultiplier tubes, holes in the beryllium window and seal failings in the outer casing. Due to the hygroscopic properties of the NaI (T1) and the special cement used to seal the assemblies together, no field repair of the instrument is possible. Therefore, repair of these instruments is expensive and time consuming.